Field of the Invention
The field of the present invention relates generally to the technical field of antennas and, more specifically, it relates to mast mountable antennas that function as rugged, weather-tolerant, vertically polarized, outdoor antennas unaffected by the mounting structure while maintaining a low impedance electrical path for lightning surge currents.
Background
RF Currents Along Masts and Mounting Structures
Unintentional radio frequency currents flowing along, for example, the feed line, mast, and mounting structure, cause harm to the desired radiation pattern of an antenna assembly. A functional antenna design incorporates design features that allow radio frequency current to flow in such a way to maximize radiation of electromagnetic energy in the desired directions. A successful antenna design not only arranges conductors to provide the primary radiation mechanism, but includes design features to ensure the radio frequency current does not flow to other portions of the antenna system including, for example, the feed line, mast, and mounting structure.
Lightning Surge Management
In recent years, the susceptibility of radio communications facilities to induced and conducted lightning surge currents has increased as radio equipment manufacturers place more burden of protection on the facility buildings, infrastructure, and antenna systems.
The research to date indicates that every part of a communications system shall be designed to tolerate and handle the damaging currents from a near or direct lightning strike. Various industry standards and guidelines provide advice on infrastructure requirements including the proper use of robust bonding techniques for all the communications components via low inductance, high current capacity conductors all connected to earth grounds. The Motorola R56 document “Standards and Guidelines for Communication Sites,” for example, is one publication specifying and teaching such techniques.
A properly designed antenna minimizes induction and conduction of radio frequency currents along, for example, antenna masts, antenna feed lines, and antenna mounting structures, while maximizing the conduction of lower frequency lightning surge currents from the antenna structure to the antenna mounting structure or grounding system. Moreover, a properly designed antenna prevents the accumulation of triboelectric charge on any part of the antenna assembly and directs any such charges to the mounting structure.
Previous Antenna Techniques
In 1909 in German Patent No. 225,204 (“Aerial Ladder Structure for Airships”), Beggerow illustrates the proto J antenna using link coupling as galvanic isolation of the aerial from the airship frame while passing radio frequency currents via the coupling's mutual inductance. Beggerow's system omits a mechanism to prevent the accumulation of triboelectric charge between the antenna conductor and the airship airframe.
In U.S. Pat. No. 2,124,424 (“Antenna System”), Leeds describes an antenna in the more familiar upright J antenna shape providing a continuous and robust conductor from top to bottom with no insulating sections, while facilitating half-wave aerial functionality at a radio operating frequency. Leeds discusses methods to mitigate unequal loading on the transmission line connection point to avert the flow of radio frequency current along the exterior of the feed line coaxial cable. Leeds omits discussion of the situation where the transmission line exterior conducting surface loads the feed point with an arbitrary impedance in parallel with the impedance presented by the antenna system depending only on the installation circumstances of the feed line.
The “J Antenna” described by the United States War Department in its 1943 technical manual number TM 11-314 (entitled “Antennas and Antenna Systems”), highlights the utility of using an attached conductive mast as a means to convey direct current and low frequency lightning surge currents to earth, but fails to recognize the necessity to isolate the mast at the antenna operating frequency.
In U.S. Pat. No. 4,208,662 (“Omnidirectional, Vertically Polarized Antenna”), Horn et al. show a method of mitigating coupling of energy from the radials by bending radial elements downward. Despite their efforts, the mechanical antenna element topology disregards the induction effects that, regardless of the antenna topology, excite currents in the supporting mast structure.
In U.S. Pat. No. 4,259,673 (“Stub Matched Antenna and Method of Feeding Same”), Guretzky outlines a method of coupling radio frequency energy from a feed line to a J antenna by wrapping the insulated center conductor of the feed line around the antenna element. Guretzky's technique lacks a means to provide a robust direct current short between the center and shield of said feed line to mitigate effects of lightning and triboelectric static charge.
In U.S. Pat. No. 4,282,531 (“Vertical Antenna with Upwardly Flaring Base Mounted Conductors”), Blaese shows a method of energizing an end fed dipole with a lower quarter-wave section made with three upward pointing conductors, but he lacks a means to mitigate the flow of radio frequency current on the antenna feed line and antenna supporting mast.
In U.S. Pat. No. 4,352,109 (“End Supportable Dipole Antenna”), Reynolds et al. introduce a secondary means to isolate the mast mounting structure, but fail to provide a direct current path for lightning currents to ground; and the methods described do not provide a robust direct current short between the center conductor and shield of the coaxial transmission line.
In the March 1998 article “The J-Pole Revisited,” Richardson identifies and mitigates both the mast and feed line radio frequency currents with an inline mast insulator and feed line coil choke, respectively, resulting in a functional antenna immune to the radio frequency current flow induced by the impedances presented in parallel to the antenna impedance by the mast and feed line. The approach does not, however, provide a robust direct current path for lightning surge currents.
In the October 2000 article “A 146- and 445-MHz J-Pole Antenna,” Griffith teaches a modification to the traditional J antenna design whereby the traditional feed point on the lower “J” stub is removed leaving the stub to, allegedly, perform the role of a mast decoupling stub. The feed point is moved to a split upper half-wave element, comprising a vertical center-fed dipole, and fed at the split with a feed line internal to the hollow antenna structure. Other design amendments facilitate additional operation at, approximately, the third harmonic of the primary frequency. While introducing new functionality to the basic J antenna design, it neglects to provide a robust direct current short between the feed line center conductor and shield, and fails to provide a robust direct current path for lightning surge currents from the top of the antenna to the earth. Additionally the alleged use of the lower traditional J element as a mast decoupling stub fails in view of the fact that the J stub's position with respect to the upper vertical dipole provides no radio frequency current choking action and instead transforms the high impedance bottom end of the above vertical dipole to a low impedance at the bottom of the J stub, thereby allowing radio frequency currents to propagate through the J section and to the conductive mast below.
In a July 2005 forum post on www.eham.net titled “Decoupling Radials on Elevated Verticals,” Hunt states that one-quarter wavelength radials are placed one-quarter wavelength below the point where one would want to block the flow of RF current, recognizing that at the point where the one-quarter wave radials intersect with the mast, the impedance is a relatively low value. This relatively low impedance is transformed via the inline conductive mast to relatively high impedance one-quarter wavelength above or one-quarter wavelength below the connection point. He describes the usage of these mast decoupling radials one-quarter wavelength below the radials of a traditional vertical antenna while focusing on the fact that each set of radials performs a distinctly separate function in the overall antenna and antenna mast design. Hunt continues with a discussion of the possible interactions between the two sets of radials and how interaction is mitigated by antenna designers using differing numbers of radials for each set. This explanation does not address the concept of orienting a single lower radial upward and parallel to the mast to accomplish the same mast decoupling role without protruding into horizontal space away from the mast.
In U.S. Pat. No. 7,859,477 (“J-Pole Antenna”), Birnbaum et al. reveal a way to connect a J antenna directly to a coaxial connector, but fail to provide a means to mitigate the flow of currents along the outside of the implied transmission line connected to the coaxial connector.
In U.S. Pat. No. 8,947,313 (“Radial-Free Collinear Omni-Directional Antenna with Gain and Virtual Ground”), Fong describes a means to make a collinear antenna using the basic J antenna as a fundamental building block, but provides no means to mitigate radio currents on a conductive mounting mast and feed line.
In U.S. Pat. No. 8,593,363 (“End-Fed Sleeve Dipole Antenna Comprising a 3/4-Wave Transformer”), McLean et al. provide a method to mitigate the conduction of radio frequency currents along the mast mounting structure. However, the antenna design lacks a means to provide a direct current short from the uppermost radiating element to the mounting mast to properly manage lightning surge currents.
In a July 2010 post to www.rec.radio.amateur.antenna, Duffy describes a method of dressing the feed line externally to yield an integral radio frequency choke action. However, he lacks a means to provide a method to place the feed line within the hollow pipe structure to protect the feed line from the elements.